Alpha Gamma Coactivation

• Gamma Motor Neurones pt1:

  • alpha motor neurones supply the extrafusal fibres
  • gamma motor neurones supply the intrafusal fibres
  • dynamic gamma supplies the adaptive fibres (dynamic bag)
  • static gamma supplies the nonadaptive fibres (static bag, chain)
  • separate motor neurones for adaptive fibres and nonadaptive fibres allow us to separately tension one fibre and not the other, so we can alter the sensitivity of these different components separately

  
• Gamma Motor Neurones pt2:

  • if dynamic gamma neurones caused tension in all dynamic bags, the spindle will be more sensitive to speed of stretch
  • if static gamma neurones caused tension in all static bags and chain fibres, the spindle will be more sensitive to the length of stretch

  
• Alpha Gamma Coactivation:

  • gamma motor neurones ensure muscle spindle is always at a level of tension that will permit it to respond to stretch - if theres no gamma motor neurones, when muscles shorten, 2 ends of the spindle would be brought closer together, taking tension off intrafusal fibres, causing no action potentials and no information about length of a muscle being generated
  • whenever muscle shorten, gamma motor neurones ensure that intrafusal fibres will shorten accordingly, so the fibres are perfectly tensioned for the length of a muscle so they always remain responsive to stretch

  
• Alpha Gamma Coactivation:

  • whenever there is input to the alpha motor neurone, there will also always be branches of the axon supplying the gamma motor neurones
  • so whenever action potentials come down to the alpha motor neurone, there will also be action potentials arriving at the gamma motor neurone - they will be coactivated
  • so whenever alpha motor neurone causes muscle to shorten, the gamma motor neurone will cause spindle to shorten accordingly, making sure the spindle is always at perfect tension to be responsive to any stretch

  
• Alpha Gamma Coactivation:

  • only works one way - gamma motor neurones activated when alpha motor neurones are activated, not the other way round, as gamma motor neurones can work independently to change level of tension in spindle to alter the sensitivity